1. Field of the Invention
The present invention relates to a piezoelectric actuator and a method for producing the piezoelectric actuator.
2. Description of the Related Art
As a conventional ink-jet head, there is known an ink-jet head having a cavity unit which is constructed by stacking a plurality of sheets and which has a plurality of pressure chambers formed and aligned in rows therein, and a piezoelectric actuator which has active portions (energy-generating mechanism) corresponding to the pressure chambers respectively and which is joined to the cavity unit. For example, as shown in FIG. 4 of U.S. Patent Application Publication No. US2005/162484A1 and FIG. 4 of U.S. Patent Application Publication No. US2005/248628 (corresponding to FIG. 4 of Japanese Patent Application Laid-open No. 2006-15539), there is known a piezoelectric actuator including a stack-portion in which ceramic sheets each having a pattern of individual electrodes is formed on a surface thereof and ceramic sheets each having a pattern of a common electrode formed on a surface thereof are stacked alternately, and a top ceramic sheet which is arranged above the stack-portion and which has surface electrodes for the individual electrodes (individual-surface electrodes) and surface electrodes for the common electrode (common-surface electrodes) formed on a surface thereof; wherein these ceramic sheets are calcinated to be integrated, and the surface electrodes for the common electrode are formed to be elongated on the surface of the top ceramic sheet.
In such a piezoelectric actuator, the surface electrodes for the common electrode are formed on the surface of the top ceramic sheet at both end portions in a row direction of the surface electrodes for the individual electrodes, and the surface electrodes for the common electrode are formed in a direction orthogonal to the row direction.
In the recent years, there is a tendency that the number of the nozzles is increased for increasing the recording speed and for realizing higher resolution. As the number of nozzles is increased, the number of surface electrodes for the individual electrodes is increased as well. With this, the electric current passing in the surface electrodes for the common electrode are increased, which in turn makes it necessary to secure a larger area for the surface electrodes for the common electrode. As a result, the surface electrodes for the common electrode are formed to be elongated on the surface of the top ceramic sheet.
A material forming the surface electrodes for the common electrode has a thermal shrinkage greater than that of a material for forming the top ceramic sheet. Therefore, when the surface electrodes for the common electrode are formed to be elongated on the top ceramic sheet, and when the top ceramic sheet is subjected to the calcination as described above to be integrated to form the piezoelectric actuator, there is a fear that the piezoelectric actuator is damaged or broken due to the difference in the amount of thermal shrinkage between the top ceramic sheet and the surface electrodes for the common electrode. Namely, the piezoelectric actuator is arched or warped with a side of the surface electrodes for the common electrode being the valley of the arch or warpage, and there is a fear that the piezoelectric actuator is cut in some cases. For example, in a case that a flexible flat cable is used for inputting a driving signal, it is difficult to connect connection terminals of signal lines of the flexible flat cable to the surface electrodes of an arched piezoelectric actuator, respectively. Further, upon joining the arched piezoelectric actuator to the cavity unit of the ink-jet head, it is difficult to make the piezoelectric actuator to be attached tightly with respect to the pressure chambers in a uniform manner, which results in causing ink leakage, thereby making the jetting force and/or the like to be non-uniform.
An object of the present invention is to provide a piezoelectric actuator and a method for producing the piezoelectric actuator capable of lowering the influence of the difference in the amount of thermal shrinkage between the top ceramic sheet and a surface electrode for the common electrode when the piezoelectric actuator is subjected to the calcination, thereby suppressing the arching deformation of the piezoelectric actuator to secure a predetermined flatness thereof.